Laser diode arrays, both linear (so-called laser diode bars) and bi-dimensional (2-D stack of laser bars) are very attractive sources of high power laser beam. Among their advantages, they have a high conversion efficiency, a high reliability, and they can be packaged to form small laser source modules. High-power laser diode linear arrays are mostly available in the form of elongated thin laser diode bars comprising several individual laser emitters set along an axis parallel to the semiconductor PN junction of the individual diodes. A schematic drawing of a laser diode bar is sketched in FIG. 1 (PRIOR ART). Common laser diode bars comprise typically 10 to 50 individual equally spaced emitters, which spread over a total width generally set to a 10 mm standard width. Each individual emitter has typical dimensions of 50–200 μm×1 μm, and they are represented by the small hachured rectangles in FIG. 1. As seen in FIG. 1, the divergence angles of the beam escaping from a laser emitter are typically 80° FW (full width at 1/e2) along the direction perpendicular to the junction plane (arbitrarily designated the Y axis on the enclosed drawings), also called the fast axis, and 10° FW along the direction parallel to the junction plane (the X axis), also called the slow axis. These laser diode bars can routinely emit tens of Watts of CW optical power. When even higher optical output powers are required from the laser source, several laser diode bars can be stacked one above the other using suitable mounting means to give a two-dimensional laser diode array. The mounting means is designed to hold the laser bars firmly in place while ensuring proper electrical biasing and cooling of each bar. The resulting total output power scales directly with the number of stacked laser diode bars.
Laser diode systems are used in various applications such as machining of materials, treatment of surfaces, illumination and pumping of solid-state lasers. These various applications have different requirements on the laser beam characteristics, such as the beam's shape and divergence. One major difficulty in using laser diode systems is that the unconditioned output laser beam is rarely adapted for applications of interest. Laser beam conditioning and/or reshaping is therefore required in order to optimize the output laser beam characteristics.
The characteristics of a laser beam outputted by a laser diode are usually quantified by a value called beam product parameters, or BPP, which represents the product of the beam's width by its divergence along a given axis. Typically, the output laser beam of a laser diode bar, taken as a whole, has BPP of 0.001 mm×1400 mrad=1.4 mm·mrad along the Y axis and a BPP of 10 mm×175 mrad=1750 mm·mrad along the X axis. The BPP value along the Y axis is close to the diffraction limit value. On the other hand, the large BPP value along the X axis is indicative of a poor laser beam quality with a M2 of the order of 1700 (the M2 factor should be of the order of 1 for a beam of the best possible quality). This strong asymmetry in the BPP's and large M2 value precludes the efficient focalisation of the laser beam to a small circular spot needed, for example, for injection to an optical fibre.
In the particular case of a laser diode bar with a relatively low fill factor (W/S ratio, see FIG. 1), several techniques have been devised to reshape the laser beam, as for example taught by J. Endriz, U.S. Pat. No. 5,168,401; S. Yamaguchi et al., U.S. Pat. No. 5,513,201; K. Du et al., U.S. Pat. No. 6,324,190; Kusuyana et al., U.S. Pat. No. 6,639,727; and Lissotschenko et al., U.S. Pat. No. 6,471,372.
These techniques share the same basic idea that each laser beamlet, i.e. the laser beam radiated by each emitter in the laser diode bar, is rotated by 90 degrees along the Z axis by the device. Either reflective (U.S. Pat. Nos. 5,168,401, 5,513,201 and 6,324,190) or refractive approaches (U.S. Pat. Nos. 5,513,201, 6,639,727 and 6,471,372) have been devised to produce this rotation. This is schematized in FIG. 2 (PRIOR ART), where a laser bar with five emitters is represented. The fast axis (Y axis) of the laser beam is collimated with a cylindrical microlens. Then, the five beamlets pass through the device which rotates each beamlet by 90 degrees around the Z axis. The asymmetric nature of the BPP of each beamlet is represented by a cross-section in the form of an elongated rectangle. The laser beam cross section at two different planes, before and after the device, takes the form represented in the dotted boxes of FIG. 2. Therefore, the total (sum) BPP of the beamlets are transformed so that the new values along the X and Y axes are related to the initial values along the Y and X axes, respectively, and reduce and increase, respectively, by time the number of rotated beamlet, that is BPP(X)≧initial BPP(Y)×5, and BPP(Y)=initial BPP(X)/5. Furthermore, the outputted laser beam's brightness, given by the optical power/(BPP(X)×BPP(Y)), will be optimized (i.e. the equality will hold in the equation above) if no empty space is left between the beamlets.
The major drawback of these techniques is that the beamlets need to be well separated before entering the rotating device, in order to minimize truncation of the beamlet's edges in the device. Therefore, this requirement precludes an efficient application of these techniques to laser diode bars with a high fill factor. In addition, it results in non-optimal laser beam brightness since significant empty spaces are left between the beamlets. Ideally, the beamlet cross section should have a height equal to the separation (pitch) between them so as to leave no empty space between them once the rotation is performed, thereby leading to an optimized output laser beam radiance.
For the reflective techniques, significant empty spaces between beamlets are required to provide enough space for the two or more reflective surfaces needed to realize the beamlet rotation. It is also the case for the refraction-based devices, which in practice consist of extended cylindrical lenses disposed side by side in close contact at a 45° angle with respect to the Y and X axes. Such an arrangement leads to beamlet's input/output aperture of diamond shape, such as schematized in FIG. 3A (PRIOR ART). The beamlets, generally having a square cross-section, experience a large clipping by going through such an optical device. This is exemplified in FIG. 3B (PRIOR ART), where the upper left and bottom right corners of the square cross section are cut-off by the diamond-shaped input/output aperture. To minimize the beamlet clipping, the horizontal extent of the beamlet's cross-section needs to be limited, as schematized in FIG. 3C (PRIOR ART). This however severely limits the laser diode bar fill factor that can be used with such device, therefore limiting its domain of application and interest.
There is therefore a need for a symmetrization device which alleviates the above-mentioned drawbacks.